Laser diodes are available as single mode or multimode diodes. Single mode laser diodes are effectively point sources, diffraction limited in their divergence in both axis. Multimode diodes typically have laser junctions in the form of short stripes, typically from 10 to 200 microns long. Multimode diodes are diffraction limited in the direction perpendicular to the junction but have non-diffraction limited divergence in the direction parallel to the laser junction. The emitting aperture of a multimode diode can be a single continuous stripe, a collection of short stripes or even a collection of single mode emitters electrically connected in parallel. All these difference constructions will be referred to as "multimode laser diodes". The diodes can be used in the visible part of the spectrum or in the IR. The term "light" is used here to cover all wavelengths.
Two dimensional laser diode arrays have been used for high resolution recording both in the optical data storage and laser plotting fields. These arrays make better use of the field-of-view of the lens than linear arrays. Two dimensional arrays are described in U.S. Pat. Nos. 4,743,091; 5,291,329 and 5,477,259.
All prior art requires a very large de-magnification ratios, typically over 100, to reduce the mechanical pitch of the array to an acceptable high resolution image. This high de-magnification ratio requires the distance between the final imaging lens and the collecting lenses to be typically over 100 times the focal length of the imaging lens. Such a large distance is acceptable for single mode laser diodes, as light from the point sources of single mode laser diodes can be collimated, but is too large for multimode lasers, as light from the spatially extended emitters of multimode laser diodes cannot be collimated and diverges, therefore missing the aperture of the final lens.
A second difficulty using multimode laser diodes is their non-uniform near field pattern. Even if the diodes are equally spaced mechanically, the equivalent emission point can move around by many microns as the near-field pattern changes with aging. This variation in the near-field pattern causes unacceptable banding in the image if the near field of the diodes is imaged onto the recording material. For these two reasons all prior art designs of two dimensional laser diode arrays used for recording had to use single mode diodes. Single mode diodes are more expensive than multimode diodes (for the same output power) and are limited in input power. Most prior art methods of using multimode diodes use multimode optical fibers to direct light from the laser diodes. Coupling to optical fibers causes a large brightness loss.